An Arduino board is a microcontroller whereas a Raspberry Pi is a full single board computer usually running Linux.
A program written in Processing basically just a Java program that depends heavily on the Processing libraries whereas a program for the Arduino would generally be written in C and then compiled with an AVR specific compiler to produce a binary for the microcontroller.There is no practical way to run a Processing sketch on an Arduino. If you want to write a program for your arduino you as may as well just use the Arduino IDE.That is the Arduino IDE uses C and provides additional AVR specific libraries in C and then compiles for the arduino using avr-gcc.
If you could explain exactly what the end result will be a little better that would help in answering the question. It sort of sounds like you are talking about two separate pieces, a “background” or perhaps embedded program to drive an LED strip and a separate front end program to control the behavior of the other one.
EDIT: You may just want to go with the Pi since libraries exist to drive it’s GPIO from Python and other higher level languages. You could write code for that on the Mac, but you’d only be able to test it on the Pi since your Mac doesn’t have GPIO. Also, the Pi and Arduino likely do not have not equivalent capacity to drive an LED strip on the pins.